Systems and Methods for Tethering a Subsea Structure

ABSTRACT

A system for tethering a subsea structure includes an anchor embedded in the sea bed, a mud mat disposed partially above the seafloor, a link attached to the anchor and to the mud mat, and a flexible tension member. One end of the flexible tension member is coupled to a tensioning system configured to pay in and pay out the flexible tension member. The tensioning system is mounted on one of the subsea structure or on the mud mat. The other end of the flexible tension member is coupled to the other of the subsea structure or on the mud mat. The anchor may optionally be buried below the seafloor.

BACKGROUND

The disclosure relates generally to systems and methods for tethering subsea structures. The disclosure relates more particularly to systems and methods that permit utilizing existing commercial anchors.

Known systems and methods for tethering subsea structures, such as a subsea Blow-Out Preventer (“subsea BOP”), usually include an anchor disposed about the subsea BOP and secured to the seafloor, a flexible tension member extending from a first end to a second end. The flexible tension member extends horizontally and vertically from the first end to the second end to impart a lateral preload and a vertical preload to the subsea BOP. A tensioning system may be coupled to the anchor. The first end of the flexible tension member is coupled to the tensioning system. The second end of the flexible tension member is coupled to the subsea BOP. The tensioning system permits the adjustment of the tensile preload applied to the flexible tension member.

Mooring anchors may have been deployed before the need for tethering subsea structures, such as a subsea BOP, has been realized. It may be difficult to couple a tensioning system to already deployed mooring anchors because the locations of the mooring anchors and the subsea BOP would cause the flexible tension member to interfere with other subsea infrastructures or flowlines. Furthermore, it may be difficult to deploy additional mooring anchors at new locations because the additional mooring anchors could also interfere with other subsea infrastructures or flowlines.

Standard mooring anchors, such as Suction Embedded Plate Anchors (“SEPLAs”), and torpedo piles, may be buried under the seafloor. It may be difficult to mount a tensioning system or an end of a flexible tension member to mooring anchors buried under the seafloor. Furthermore, other mooring anchors, such as suction piles, may not be easily fitted with a tensioning system or an attachment means for the end of the flexible tension member.

Thus, there is a continuing need in the art for systems and methods for tethering subsea structures. Preferably, the systems and methods would permit utilizing existing commercial anchors that are buried under the mud line.

SUMMARY OF THE DISCLOSURE

The disclosure describes a system for tethering a subsea structure.

The system may comprise an anchor embedded in the sea bed, a mud mat disposed partially above the seafloor, a link attached to a point of the anchor located below the seafloor and to the mud mat and a flexible tension member, wherein the flexible tension member extends horizontally and vertically from a first end to a second end to impart a lateral preload and a vertical preload to the subsea structure.

In some embodiments, the system may comprise a tensioning system that is mounted on the mud mat, the first end of the flexible tension member may be coupled to the tensioning system, and the second end of the flexible tension member may be coupled to the subsea structure.

In other embodiments, the system may comprise a tensioning system that is mounted on the subsea structure, the first end of the flexible tension member may be coupled to the tensioning system, and the second end of the flexible tension member is coupled to the mud mat.

The anchor may be buried below the seafloor. Alternatively, the anchor may have an upper end disposed above the seafloor and a lower end disposed in the seabed below the seafloor.

The disclosure also describes a method for tethering a subsea blow-out preventer (BOP) coupled to a subsea wellhead.

The method may comprise the steps of embedding an anchor to in the sea bed, disposing a mud mat partially above the seafloor, attaching a link to a point of the anchor located below the seafloor and to the mud mat, and applying tension to a flexible tension member to impart a lateral preload and a vertical preload to the subsea BOP.

In some embodiments, the method may further comprise the step of mounting a tensioning system on the mud mat, the tensioning system being configured to pay in and pay out the flexible tension member. The flexible tension member may extend horizontally and vertically from a first end coupled to the tensioning system to a second end coupled to the subsea BOP.

In other embodiments, the method may further comprise the step of mounting a tensioning system on the subsea BOP, the tensioning system being configured to pay in and pay out the flexible tension member. The flexible tension member may extend horizontally and vertically from a first end coupled to the tensioning system to a second end coupled to the mud mat.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the embodiments of the disclosure, reference will now be made to the accompanying drawings:

FIG. 1 is an elevation view of a system for tethering a subsea structure to a suction embedded plate anchor;

FIG. 2 is an elevation view of a system for tethering a subsea structure to a torpedo anchor;

FIG. 3 is an elevation view of a system for tethering a subsea structure to a suction pile;

FIG. 4 is a perspective view of system 10 for tethering a subsea structure that includes a plurality of mud mats;

FIG. 5 is a perspective view of an example mud mat; and

FIG. 6 is a perspective view of an example tensioning system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

FIGS. 1, 2, and 3 illustrate a system 10 for tethering a subsea structure. For example, the subsea structure may include a subsea BOP 12.

The system 10 comprises an anchor embedded in the seabed. For example, in FIG. 1 , the anchor includes a suction embedded plate anchor (“SEPLA”) 14, which is buried below the seafloor 16. In FIG. 2 , the anchor includes a torpedo anchor 18, and the anchor is also buried below the seafloor 16. In FIG. 3 , the anchor includes a suction pile 20 having an upper end disposed above the seafloor 16 and a lower end disposed in the seabed below the seafloor 16. Embedding the anchor deeper in the seabed usually increases the resistance of the anchor.

The system 10 comprises a mud mat 22 disposed partially above the seafloor 16 at a location nearer to the anchor than the subsea structure. As shown in FIG. 5 , the mud mat 22 may include a wall 24 that surrounds a horizontal plate 26, the horizontal plate 26 being located approximately half-way along the height of the wall 24. The mud mat 22 may also include vertical ribs 28 that preferably form a crisscross pattern. In use, the lower portion of the wall 24 and the lower portions of the vertical ribs 28 may sink into the seabed under the weight of the mud mat 22 until the horizontal plate 26 reaches the seafloor 16. As such, the lower portion of the wall 24 and the lower portions of the vertical ribs 28 transmit horizontal load applied to the mud mat 22 to the seabed and hinder sliding of the mud mat 22 on the seafloor 16 up to a maximum magnitude of the horizontal load, usually in the order of 40 tons. The mud mat 22 may further include one or more receptacles 24 that are sized and configured to receive and retain a stabbing member. The mud mat 22 may include one or more eyes 46, each adapted to attach a link.

The system 10 comprises a link 30 attached to a point 32 of the anchor located below the seafloor 16 and to the mud mat 22. The link 30 may include a cable, a chain, or another known link. The link 30 is preferably placed under tension between the point 32 of the anchor and the mud mat 22. As such, the link 30 can be used to hinder sliding of the mud mat 22 on the seafloor 16 under loads that exceed the maximum magnitude of horizontal load resisted by the mud mat 22 alone.

The system 10 comprises a tensioning system 34. As shown in FIG. 6 , the tensioning system 34 may include a winch 44 configured to pay in and pay out a flexible tension member. The winch 44 may be operated by a Remotely Operated Vehicle (“ROV”). The tensioning system 34 may include a stabbing member 36 that is sized and configured to be received and retained in one of the receptacles 24. As such, the tensioning system 34 may be mounted on the mud mat 22. The tensioning system 34 may be mounted on the mud mat 22 using mechanisms other than a stabbing member being received and retained in a receptacle.

The system 10 comprises a flexible tension member 38. The flexible tension member 38 extends horizontally and vertically from a first end to a second end to impart a lateral preload and a vertical preload to the subsea structure when the flexible tension member 38 is placed under tension. The tensioning system 34 is used to adjust the preload, preferably to a level sufficient to pull out any slack, curve, and catenary in the flexible tension member 38. The first end of the flexible tension member 38 is coupled to the tensioning system 34. The second end of the flexible tension member 38 is coupled to an elevated point 40 of the subsea structure, such as with a hook adapted to be opened and closed with an ROV.

The point 32 is preferably selected, and the link 30 is preferably sized, such that, in use, a center of the mud mat 22 is located on or above a plane 42 joining the point 32 to the elevated point 40. As such, the link 30 and the flexible tension member 38 would impart a downward force to the mud mat 22 when they are placed under tension.

In alternative embodiments, the tensioning system 34 may be mounted on the subsea structure, and the flexible tension member 38 may include one end coupled to the tensioning system 34 and another end coupled to the mud mat 22. Furthermore, two tensioning systems 34 may be used in conjunction, one mounted on the subsea structure, and one mounted on the mud mat 22.

FIG. 4 illustrates an embodiment of the system 10 for tethering a subsea structure. In this embodiment, the system 10 includes four mud mats 22. Each of the mud mat 22 may be coupled to one or more anchors (not shown). A single tensioning system 34 and corresponding flexible tension member 38 are coupled to each of the mud mats 22.

In other embodiments, more or less than four mud mats 22 may be used. Furthermore, more than one tensioning system 34 and corresponding flexible tension member 38 may be coupled to each of the mud mats 22. In addition, all or some of the tensioning systems 34 may be mounted on the subsea structure instead of the mud mats 22, as explained hereinabove.

In other embodiments, the system 10 may include a plurality of links, each attached to a corresponding one of a plurality of anchors embedded in the seabed. Optionally, more than one link may be attached to a single anchor. Each of the plurality of links is attached to an eye of the mud mat 22, of to other attachment means. In such embodiments, the mud mat 22 may not necessarily be in the vertical plane joining the BOP 12 and one of the anchors. Furthermore, in such embodiments, a few of the links attached to the mud mat 22 may be attached to a point of the anchor located at the seafloor 16, or slightly above the seafloor 16.

In other embodiments, a link 30 may comprise a first link portion attached to a point of a first anchor located below the seafloor 16 and attached to or wrapped on a coupler, and a second link portion attached to or wrapped on the coupler and attached to the mud mat 22. Optionally, the link 30 may comprise a third link portion attached to a point of a second anchor and attached to or wrapped on the coupler, the point of the second anchor being preferably, but not necessarily located below the seafloor 16. 

1. A system for tethering a subsea structure, the system comprising: an anchor embedded in the sea bed; a mud mat disposed partially above the seafloor; a link attached to a point of the anchor located below the seafloor and to the mud mat; a flexible tension member, wherein the flexible tension member extends horizontally and vertically from a first end to a second end to impart a lateral preload and a vertical preload to the subsea structure; and a tensioning system mounted on the mud mat, wherein the tensioning system is configured to pay in and pay out the flexible tension member, wherein the first end of the flexible tension member is coupled to the tensioning system, and wherein the second end of the flexible tension member is coupled to the subsea structure.
 2. A system for tethering a subsea structure, the system comprising: an anchor embedded in sea bed; a mud mat disposed partially above the seafloor; a link attached to a point of the anchor located below the seafloor and to the mud mad; a flexible tension member, wherein the flexible tension member extends horizontally and vertically from a first end to a second end to impart a lateral preload and a vertical preload to the subsea structure; and a tensioning system mounted on the subsea structure, wherein the tensioning system is configured to pay in and pay out the flexible tension member, wherein the first end of the flexible tension member is coupled to the tensioning system, and wherein the second end of the flexible tension member is coupled to the mud mat.
 3. The system of claim 1, wherein the anchor is buried below the seafloor.
 4. The system of claim 3, wherein the anchor includes one of a torpedo anchor and suction embedded plate anchor (SEPLA).
 5. The system of claim 1, wherein the anchor has an upper end disposed above the seafloor and a lower end disposed in the seabed below the seafloor.
 6. The system of claim 5, wherein the anchor includes a suction pile.
 7. The system of claim 1, wherein the subsea structure includes a subsea blow-out preventer (BOP).
 8. The system of claim 1, further comprising another tensioning system mounted on the subsea structure, wherein the second end of the flexible tension member is coupled to the subsea structure by attaching the second end of flexible tension member to the other tensioning system.
 9. The system of claim 2, further comprising another tensioning system mounted on the mud mat, wherein the second end of the flexible tension member is coupled to the mud mat by attaching the second end of flexible tension member to the other tensioning system.
 10. A method for tethering a subsea blow-out preventer (BOP) coupled to a subsea wellhead, the method comprising: embedding an anchor to in the sea bed; disposing a mud mat partially above the seafloor; attaching a link to a point of the anchor located below the seafloor and to the mud mat; mounting a tensioning system on the mud mat; and applying tension to a flexible tension member to impart a lateral preload and a vertical preload to the subsea BOP, wherein the flexible tension member extends horizontally and vertically from a first end coupled to the tensioning system to a second end coupled to the subsea BOP, and wherein the tensioning system is configured to pay in and pay out the flexible tension member.
 11. A method for tethering a subsea blow-out preventer (BOP) coupled to a subsea wellhead, the method comprising: embedding an anchor to in the sea bed; disposing a mud mat partially above the seafloor; attaching a link to a point of the anchor located below the seafloor and to the mud mat; mounting a tensioning system on the subsea BOP; and applying tension to a flexible tension member to impart a lateral preload and a vertical preload to the subsea BOP, wherein the flexible tension member extends horizontally and vertically from a first end coupled to the tensioning system to a second end coupled to the mud mat, and wherein the tensioning system is configured to pay in and pay out the flexible tension member.
 12. The method of claim 10, wherein the anchor is buried below the seafloor.
 13. The method of claim 10, wherein the anchor has an upper end disposed above the seafloor and a lower end disposed in the seabed below the seafloor.
 14. The system of claim 2, wherein the anchor is buried below the seafloor.
 15. The system of claim 14, wherein the anchor includes one of a torpedo anchor and suction embedded plate anchor (SEPLA).
 16. The system of claim 2, wherein the anchor has an upper end disposed above the seafloor and a lower end disposed in the seabed below the seafloor.
 17. The system of claim 16, wherein the anchor includes a suction pile.
 18. The system of claim 2, wherein the subsea structure includes a subsea blow-out preventer (BOP).
 19. The method of claim 11, wherein the anchor is buried below the seafloor.
 20. The method of claim 11, wherein the anchor has an upper end disposed above the seafloor and a lower end disposed in the seabed below the seafloor. 